Passage II

Disinfection is a process in which harmful bacteria and viruses are destroyed. Two scientists present their opinions on disinfection of water by chlorine.

Scientist 1

Chlorine (Cl₂) should not be used to disinfect public water supplies. Water supplies contain bacteria and viruses as well as natural organic matter. Organic by-products are formed when chemical reactions between Cl₂ and natural organic matter occur. These by-products should not be in public water systems because they are hazardous and can be toxic to humans.

Any Cl₂ left in water after disinfection is then consumed by the public. The excess Cl₂ can react with the components of saliva, stomach fluid, and food, thus forming new by-products which could also be toxic.

There are alternatives to Cl₂ for the disinfection of drinking water. Chlorine dioxide and chloramine are milder forms of chlorine that could be used to disinfect water. Ozone is another possible disinfectant. The use of ultraviolet (UV) radiation to disinfect water has also been found to kill viruses more effectively than Cl₂.

Scientist 2

Chlorine should be used to disinfect public water supplies. Chlorine can, in many cases, be more effective than other methods of disinfection. Chloramine will effectively kill bacteria, but is much less effective than Cl₂ in destroying viruses. UV radiation can inactivate bacteria and viruses, but in some cases the regrowth of bacteria has been found.

There is concern that organic by-products are formed by the reaction of Cl₂ with natural organic matter in the water. However, there are ways to remove these by-products. One is by first removing the natural organic matter before it reacts with the Cl₂. This is accomplished by an extra treatment step such as filtering the water beforet he addition of Cl₂. Subsequent addition of sodium sulfite to the Cl₂-treated water will remove or destroy any excess by-products.

In addition to killing bacteria and viruses, Cl₂ will also react with some undesirable pesticides that may be present in the water already. These reactions can cause the amount of pesticides in the water to decrease.